Speed control system for a rolling mill



Nov. 17, 1970 R. s.HosTET1-ER ETAL 3,540,248

SPEED CONTROL SYSTEM FOR A ROLLING MILL Filed July 18. 1968 United States Patent O 3,540,248 SPEED CONTROL SYSTEM FOR A ROLLING MILL Richard S. Hostetter, William .Ieuick, Harold A. List, and Wolfgang M. Sawitz, Bethlehem, Pa., assignors to Bethlehem Steel Corporation, a corporation of Delaware Filed `Iuly 18, 1968, Ser. No. 745,834 Int. Cl. B21!) 37/10, 37/00 U.S. Cl. 72-8 3 Claims ABSTRACT F THE DISCLOSURE A rolling mill reduces the cross section of a rod in two stands simultaneously. The speed of the rolls in the second stand is coordinated with the speed of the rolls in the first stand by a control system utilizing a first signal indicative of the armature current of the motor driving the rolls of the first stand and a second signal indicative of the temperature of the rod as it passes through said first stand.

BACKGROUND OF THE INVENTION This invention relates to rolling mills, and more particularly to a control system for coordinating the speed of a second set of rolls with the speed of a first set of rolls.

In the rolling of a workpiece wherein the cross section of said workpiece is simultaneously reduced in two stands, it is essential for the speed of the rolls in one stand to be coordinated with the speed of the rolls in the other whereby the tension or compression of that portion of the workpiece disposed between said stands can be controlled. For example, if the second stand rolls are rotating too fast, the workpiece will be in tension and will tend to neck, while if the second stand rolls are rotating too slowly, the workpiece will be in compression and will tend to buckle. Both of these conditions may adversely affect the dimensions of the workpiece.

Mills which roll workpieces of relatively small crosssection, e.g., bars less than 1.5 inches in diameter, are generally equipped with automatic looping devices. However, for larger cross sections, looping devices cannot be used. For this reason, rolling mills adapted to roll workpieces of such larger cross-sections are generally constructed so that the workpiece never passes through two stands simultaneously.

In recent years, speed coordinating systems have been designed which utilize changes in the armature current of a first motor driving the rolls of a first stand to coordinate the speed of said rolls with the speed of the rolls of a second stand through which the workpiece is simultaneously passing. Such a system is shown in U.S. Pat. No. 3,363,441 to Smith, issued Jan. 16, 1968. In the Smith system, a signal indicative of the armature current of said first motor is stored while the workpiece is passing through said first stand but prior to workpiece entry into said second stand. This signal is then compared with a second signal indicative of said armature current while said workpiece is in both the first and the second stands. If the rolls of the first stand are rotating too fast relative to the speed of the rolls of the second stand, said armature current will increase while the workpiece is passing through both stands, as the rolls in the first stand are pushing the workpiece through the second stand, thereby increasing the first stands motor load. If the rolls of the first stand are rotating too slowly relative to the speed of the rolls of the second stand, said armature current will decrease while the workpiece is passing through both stands, as the rolls in the second stand are pulling the workpiece through said first stand, thereby decreasing its motor load.

The above-described system has proved unsatisfactory 3,540,248 Patented Nov. 17, 1970 'ice We have discovered that the armature current of a motor driving the rolls of a rolling mill stand may vary by as much as 25% if the temperature of a workpiece passing through said stand changes by a relatively small amount. If, for example, the temperature of the workpiece at the `first stand increases from 1600 to 1700 F., the armature current of the motor driving the rolls of said first stand may decrease by 25%, as such an increase in temperature decreases the amount of force required to reduce the cross-section of the workpiece, and hence reduces the motor load. In a system such as the abovedescribed Smith system, such a decrease in armature current while the workpiece is passing through both stands would result in a signal indicating that the speed of the rolls in the first stand was too slow, and this signal would be utilized to increase the speed of said rolls.

We have discovered that temperature variations along the length of a workpiece can 'be compensated for by providing a system comprising means for producing a first signal proportional to the armature current in the motor driving the rolls in the first stand, means for producing a second signal proportional to the temperature of said workpiece as it passes through said iirst stand, means for producing a third signal proportional to the difference between the sum of said first and second signals Awhile said workpiece is passing through said first stand but prior to workpiece entry into a second stand and the sum of said first and second signals while said workpiece is passing through both said first and second stands, and means responsive to said third signal for adjusting the speed of the rolls in one of said stands to reduce said third signal to a desired magnitude.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic of the control system of the invention.

FIG. 2 is a schematic of the sequencing circuit utilized in the control system of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT As is shown in FIG. l, a first roll stand 10 houses a set of vertical work rolls 12 and a second roll stand 14 houses a set of horizontal work rolls 16. Said rolls are provided with grooves 18 and 20, respectively, through which a round bar 22 is adapted to pass during a reducing operation.

A first motor 24 is provided for driving the rolls 12 and a second motor 26 is provided for driving the rolls 16. The armature circuit of the motor 24 is provided with terminals 28 and 30 which are connected to the mill power bus. A shunt resistor 32 is included in the armature circuit and a current sensor 34, which may comprise a magnetic amplifier, is connected thereacross. The current sensor 34 is provided with an output terminal 36 which supplies a current proportional to the armature current in motor 24 to a bistable amplifier 38. The bi- -stable amplifier 38 is adapted to actuate a relay 40 when the current supplied thereto reaches a predetermined level. The relay 40 is provided with normally open contacts 42 and normally closed contacts 44, the function of which will be later described.

Disposed adjacent to the entry side of rolls 16 is a proximity detector 46, which may be a conventional hot metal detector, which is adapted to actuate a relay 48 when the bar 22 is about to enter said rolls 16. The relay 48 is provided with normally open contacts 50 and 52.

The relay contacts 42, 50 and 52 are included in a sequencing circuit 53, shown in FIG. 2, having terminals 54 and 56 connected to a suitable power supply. Said circuit comprises relays 58, V60 and 62 and timers 64 and 66. Relay 58 is provided with normally closed contacts 68, 70, 72 and 74. Relay 60 is provided with normally open contacts 76, 78, 80, 82 and 84 and normally closed contacts 86. Relay 62 is provided with normally closed contacts 88.

A temperature sensor 90, which mayY comprise an optical pyrometer, is disposed so as to produce a current inversely proportional to the temperature of the rod as it is passing through the roll stand 10. The output from the temperature sensor 90 is supplied to a track and store circuit 92. Said circuit, which is of standard design, is adapted to produce an output signal proportional to the difference between a stored reference signal and another, subsequent signal. Similarly, the terminal 36 of the current sensor 34 is connected to a track and store circuit 94 which produces an output signal proportional to the difference between a stored reference signal and another, subsequent signal.

The outputs from the track and store circuits 92 and 94 are supplied to a summing amplifier 96 which produces an output signal proportional to the sum of any differences between the above-described reference signals and subsequent signals. The output from the amplifier 96 is supplied, through contacts 84 (when closed), to a contact-type controller 98 which produces a suitable signal for controlling a motor 100 which drives the slide wire of a rheostat controlling the current in the iield 102 of motor 26.

The controller 98 is connected to a switching device 104 which prevents the controller 98 from effecting changes in the current in the field 102 when either contacts 44 or 88 are closed. When either of said contacts are closed, the iield current may only be changed manually by the rolling mill operator. Both of said contacts are open when there is a bar in both stands and 14 simultaneously.

The above-described speed control system operates substantially as follows. When the bar 22 enters the stand 10 the load on the motor 24 increases, resulting in an increase in the armature current in said motor 24. This increase in current causes the bistable amplifier 38 to actuate the relay 40 which opens contacts 44, which are connected to the switching device 104, and closes contacts 42 in the sequencing circuit 53.

The current sensor 34 supplies a signal proportional to the armature current in motor 24 to the track and -store circuit 94. Said circuit initially tracks said signal by reason of closed contacts '68 and 70. Similarly, the temperature sensor 90 supplies a signal proportional 'to the temperature of the bar 22 as it passes through stand 10 to track and store circuit 92. Said circuit initially tracks said signal by reason of closed contacts 72 and 74.

The proximity detector 46 senses the bar 22 as it is about to enter the stand 14 and actuates relay 48, which results in the closing of contacts 50 and 52 in the sequencing circuit 53.

The closing of contacts 50 initiates timer 66, which actuates relay 62 after a time elapse of about 0.5 second. During this time elapse, the bar 22 enters the stand 14, resulting in a change in the armature current in motor 24 if there is either tension or compression in that portion of the workpiece extending between said stands. This change in armature current is accompanied by transients which would adversely affect the rolling operation if allowed to actuate the controller. Said transients expire within the time elapse, after which the actuation of relay l62 opens contacts 88, causing the switching d'evice 104 to switch the controller 98 into the automatic mode.

Inasmuch as contacts 42 are closed as a result of a workpiece passing through the rolls 12, the closing of contacts 52 actuates relay 58 and initiates timer 64. The actuation of relay 58 opens contacts 68 and 70 in the track and store circuit 94 and contacts 72 and 74 in the track and store circuit 92, thereby Storing as references a first signal proportional to the armature current in motor 24 just prior to the entry of the bar into the second stand 14 and a second signal proportional to the temperature of the bar as it passes through the first stand 10, said second signal being stored at the same instant as said first signal.

The timer 64 actuates relay 60 after a time elapse of about 0.1 second, at which time the bar 22 is between the rolls 16 of stand 14. The actuation of the relay 60 closes contacts 76 and 78 in the track and store circuit 94, closes contacts 80 and 82 in the track and store circuit `92, and closes contacts 84 and opens contacts 86 at the output of the summing amplifier 96. Closing of contacts 76 and 78 results in the application of a signal to the input of the summing amplifier 96 proportional to the difference between the armature current of rnotor 24 when the bar 22 is passing through the first stand 10` only and the armature current when the bar 22 is passing through both the first stand 10 and the second stand 14. Closing of contacts 80 and 82 results in the application of a signal to the input of the summing amplifier 96 proportional to the difference between the temperature of the bar 22 at the rolls 12 at the time the reference armature current signal is stored and the temperature of the bar when it is passing through both the iirst stand 10 and the second stand 14. The closing of contacts 84 and opening of contacts 86 permits the output from the summing ampliiier 96 to be applied to the input of the controller 98. (Initially, the contacts 84 were open and the contacts 86 were closed to keep the input to the controller grounded and thus prevent feedback from the controller to the summing amplifier 96 until after the reference signals have been stored.)

The controller 98 is of the three-function type, and functions to control the direction in which iield control motor 100 drives the slide wire of a rheostat controlling the current in the eld 102 of motor 26. The controller 98 supplies the current to the motor 100 until the input to the controller is zero, at which time the speeds of the motors 24 and 26 are coordinated. The slide wire of said rheostat then remains in its final position, and the speed of the rolls is thereby substantially correct for subsequent rods passing through the mill.

By coordinating the speed of the motors is meant adjusting the speeds of the motors 24 and 26 until the proper tension or compression exists in that portion of the workpiece which extends between the two stands. While in certain cases no tension or compression may be desired, it has been found that truer dimensions can be obtained if round bars are rolled under compression and square bars are rolled under tension. Suitable bias circuits (not shown) can be added to the subject system to provide such compressison or tension.

After the bar 22 leaves the rolls `12 of stand 10, the signal supplied to the bistable amplifier decreases in response to the decreased load on the motor 24. The relay 40 is thereby deactuated, resulting in the opening of contacts 42 and the closing of contacts 44. Opening of contacts 42 deactuates relays 58 and 60, while the closing of contacts 44 switches the system from automatic to manual. As the trailing end of the bar 22 enters the rolls 16 of stand 14, the proximity detector 46 deactuates relay 48, which opens contacts 50 in the sequencing circuit and deactuates relay `62. The subject system is then ready to receive another bar, the above sequence of steps being repeated for each bar.

While the subject system has been described with reference to two stands which simultaneously reduce the cross section of a workpiece, it is equally applicable to rolling mills comprising more than two stands. In this case, the speed of the rolls last to receive the workpiece must be coordinated with the speed of the previous set of rolls, while in the case of only two stands the speeds of either set of rolls can be coordinated with the speed of the other.

We claim:

1. In a rolling lmill wherein the cross section of a workpiece is simultaneously reduced in a first and a second stand, said first stand comprising a first set of rolls driven by a first motor and a second set of rolls driven by a second motor, means for coordinating the speed of said second set of rolls with said first set of rolls comprising:

(a) means for producing a first signal proportional to the armature current in said first motor,

(b) `means for producing a second signal proportional to the temperature of said workpiece as it passes through said first stand,

(c) means for producing a third signal proportional to the difference between the sum of said first and second signals while said workpiece is passing through said first stand but prior to workpiece entry into said second stand and the sum of said first and second signals While said workpiece is passing through both said first and said second stands, and

(d) means responsive to said third signal for adjusting the drive speed of one of said motors to reduce said third signal to a desired magnitude.

2. Apparatus as recited in claim 1, in which means (c) comprises:

(i) means for storing said irst and second signals at an instant While said workpiece is passing through said first stand but prior to workpiece entry into said second stand,

(ii) means for producing a third signal indicative of the sum of the difference between said first signal at said instant and at subsequent instances after said workpiece has entered said second stand and the difference between said second signal at said instant and at subsequent instances after said workpiece has entered said second stand.

3. Apparatus as recided in claim 2, in which means (d) comprises means for adjusting the field current of said second motor.

References Cited UNITED STATES PATENTS 2,343,392 3/1944 Whitten 72-13 3,363,441 1/1968 Smith 72,-11 X 3,418,834 12/1968 Cook 72-13 MILTON S. MEHR, Primary Examiner U.S. Cl. X.R. 72-11, 13, 19 

